1. Field of the Invention
This invention relates to fuel oil and more particularly to water in oil emulsions for fuel.
2. State of the Prior Art
Conservation of energy is a national goal and any improvement of combustion processes is obviously of major interest. It has been found that water-in-oil emulsions can improve the burning characteristics of boilers, gas turbines and internal combustion engines. The advantages of this type of fuel are:
reduction of smoke and NO.sub.x emissions; PA1 savings of fuel, on the order of 5%; PA1 less fouling of boiler heat exchange surfaces; PA1 utilization of heavier or less expensive fuels.
The idea of adding water to combustion was mentioned 200 years ago, and adding emulsions to fuel about 80 years ago. During World War II, water was injected into airplane engines for a brief moment in order to obtain a burst of speed. However, in 1947 the addition of finely atomized water to spark ignition engines was recognized as a method of eliminating hard knock or premature detonation. The first evaluation of emulsions was performed on diesel engines in 1952. Recently the focus on energy shortage has hastened emulsion research and two national symposiums have been held by the United States Department of Transportation on emulsified fuels in combustion.
One difficulty in making a useful emulsion is the high requirement of homogenizing energy needed to produce a suitably small particle size, and an emulsion that will not coalesce and "cream out" (emulsion break and settle). This energy can be applied by high speed propeller, ultrasonic probe, vibrating reed or high pressure orifice. For research purposes the fuel oil emulsion is often made in a batch arrangement; a high speed blade homogenizer is typically used. However, in a practical combustion situation, when processing a continuous flow fuel line, homogenizing intensities needed must be even higher than in "beaker processing," since application of energy to any portion of a flowing stream is momentary. With diesel engine fuel in particular, intensities necessary border on the impractical because Diesel #2 oil is inherently difficult to emulsify. Its interfacial tension with water is greater than gasoline, or #4 and #6 oil. When using ultrasonics, cavitation intensity (bubble collapse force) is low because of high vapor pressure and low viscosity (low molecular attraction). Moreover, the higher fuel oil temperature (due to recirculation from the diesel engine) exacerbates the difficulty in making and maintaining the emulsion. Wetting agents or emulsifiers may be added to the mixture, and these are a big aid in producing and stabilizing an emulsion. But, because these additives are expensive and also add an extra step in the process, it is generally preferred to avoid them if possible.
The addition of water to the combustion process has several interesting effects. The most obvious is cooling. Lower temperatures reduce NO.sub.x formation; and in engines, reduce exhaust valve burning and cylinder wall losses. It is not known, however, exactly how water improves combustion efficiency. The most widely considered theory is that of "microexplosions." A properly formulated emulsion will produce a droplet explosion of sufficient violence to produce secondary atomization and micro-mixing. This occurs because the entrained water becomes superheated. The new droplets are more homogeneous and air and fuel mixing is improved requiring less excess air.
There is another more recent theory to explain the benefits of emulsion burning. Water, alcohol, or any other compound which adds more hydrogenthan carbon to the combustion process may produce the same desired results. The water emulsion then, only distributes the hydrogen evenly. The increased hydrogen theory is somewhat borne out by the fact that gasoline engines get increased efficiency from added alsohol as well as water. Since hydrogen in water does not dissociate at normal combustion temperatures, the chemistry explaining this phenomenon is not apparent. Further, engines may work better with increased water, not due to the emulsion as such, but because part of the action is now that of a steam engine which may increase the expansive output per unit of oil fuel. The cooling effect itself of the water might be the important factor; burning is more even with water vapor surrounding the burning oil droplet and less soot is formed.